Oroville spillway's drains & non-modern tech & with no waterstop slab seams have operated as a high "underslab" waterflow structure from early on. Plenty of water, pressurized while the spillway is active, penetrates and flows under the slab. Oroville's current design depends on strong anchor bars, emplaced into rock, to counter "uplift" forces from "hydraulic jacking". Now that the current set of emergency repairs on the spillway have been done, the sidewall drain outlets should give an indication of an improvement to limiting the underslab water penetration.
Below are comparison images of sidewall drain flows from 1968 while operating at 150,000 cfs. The newer picture has sidewall drain flows from 2017 while operating at 100,000 cfs (after the blowout). Both are of the upper section of the spillway near the slope progression.
Last two images are discussion material: Evidence that the Drains have an "escape" flow path under the drainpipe from the bell coupling offset of the body of the main pipe. There is evidence that the drain pipe was also placed upon gravel as the damaged portion shows concrete mortar that has imbedded material (round rock) + a "void seam" below this. This "void seam" may have been from compacted gravel to facilitate a "grade" surface to place the drain pipe upon & keep it from fulcrum pressure points on bare bedrock irregularities. The "uncaptured" subsurface waterflow would pose an erosion condition to any path that would facilitate material migration. A side point is that any "fines" (fine grain material) would be able to migrate through the perforated drain pipe holes as the drain system was not filtered.
Oroville spillway 1968 operating at 150,000 cfs (DWR history photo). Notice the volume of sidewall drain flow in this "newly" constructed timeline.
Oroville spillway 2017 operating at 100,000 cfs after "blowout failure". Both images are near the slope curvature progression of the spillway.
"In the upper slab drain design: Underflow graphic (simplified as hydraulic operating pressure has "uplift" into the drain pipe) - note the un-captured water "underflow access" potential.
Evidence of an erodible layer under the drain pipe & slab - "seam and voids" present. Potential "underflow" regions from "un-captured water" & erosion of "fines" & other pressurized movement of non-solid material
